What is "i-mutation"

n. (context linguistics English) A type of sound change in which a vowel's place of articulation is raised (becomes more close) under the influence of a following (IPAchar: /i/), (IPAchar: /j/) or similar sound.

I-mutation (also known as umlaut, front mutation, i-umlaut, i/j-mutation or i/j-umlaut) is a type of sound change in which a back vowel is fronted, or a front vowel is raised, if the following syllable contains /i/, /ī/ or /j/ (a voiced palatal approximant, sometimes called yod, the sound of English in yes). It is a category of regressive metaphony, or vowel harmony.

The term is usually used by scholars of the Germanic languages: i-mutation is particularly important in the history of the Germanic languages because inflectional suffixes containing an /i/ or /j/ led to many vowel alternations that are important in the morphology of these languages today: see Germanic umlaut.